The nature of the embryonic and perinatal development of the complex patterns of nerve cell interconnections is only beginning to be understood. Anatomical studies of the visual system of small animals which are born in a very immature state have been especially helpful in advancing this kind of knowledge. However, even for the development of the optic tract, we still know little about how single nerve fibers change progressively from simple elongating processes to adult axons with elaborate end arbors in each of several target areas. Recent work has led to the definition of separate modes of axon growth (elongation and arborization) in the developing optic tract, with a sharp transition between these modes in the lateral geniculate nucleus (LGd) of the thalamus, the major link to the visual neocortex. In the work proposed, both new and classical histological methods will be used to work out a detailed timetable of the arrival and subsequent ingrowth of several populations of visual-system axons in the LGd, and in two other important structures in this system (the superior colliculus and the lateral-posterior nucleus). Of special interest will be how these axons express different modes of growth, and how an early stage of widespread branching becomes transformed as the projections come to represent precise "maps" of the receptor surface in the retina (and hence, the visual fields). Further work will employ the electron microscope to look for ultrastructural clues to the nature of the dynamic structural transformations that appear in light-microscopic studies. Here, the nature of cell-to-cell contacts as well as intracellular changes will be described at crucial stages of optic-tract morphogenesis. In addition, study of molecular mechanisms underlying the modes of growth will be initiated in a new collaborative venture designed to find developmental changes in proteins transported by retinal ganglion-cell axons.